This invention relates to a multipath canceller for use in cancelling a distortion to which a pulse amplitude modulated radio frequency signal is subjected as a result of transmission through a plurality of transmission paths.
As will later be described more in detail, a vehicle active or a vehicle passive radio navigation system comprises an interrogator and a transponder. Each of the interrogator and the transponder comprises a transmitter and a receiver. A typical one of the radio navigation systems is TACAN/DME, namely, a distance measuring equipment (DME) of a tactical air navigation system which is generally known as a TACAN system. The interrogator transmits a pulse amplitude modulated radio frequency signal to the transponder as an interrogation signal. Responsive to the interrogation signal, the transponder transmits another pulse amplitude modulated signal back to the interrogator as a response signal. The interrogation and the response signals are usually in different radio frequency bands.
Such a pulse amplitude modulated radio frequency signal is produced by amplitude modulating a carrier signal of a radio frequency by a modulating pulse into an original radio frequency pulse of an original pulse shape. Preferably, the original pulse shape has a distribution of instantaneous amplitudes substantially according to the Gaussian or normal distribution known in statistics. The original pulse shape has a finite pulse duration or width as will later be described more in detail.
The original radio frequency pulse is received at the transponder or the interrogator inevitably through a plurality of transmission paths due to topographic and other influences. In other words, the radio frequency pulse is unavoidably subjected to multipath transmission. One of the transmission paths is a shortest or direct path. Others of the transmission paths are undesired paths. The multipath transmission gives a distortion to the original pulse shape during transmission of the radio frequency pulse through the transmission paths, namely, before reception of the radio frequency signal. A distorted radio frequency pulse of a distorted pulse shape is therefore received at the transponder or the interrogator. The distorted radio frequency pulse has a longer pulse duration than the finite pulse duration and is therefore placed at a distorted pulse position which is different from a correct pulse position which the original radio frequency pulse would have if received through the shortest path alone.
It is possible to understand that the distortion is caused in the original radio frequency pulse received through the shortest path by the original radio frequency pulse received through the undesired paths. The distortion varies depending on the undesired paths. It is therefore desirable in carrying out measurements by the radio navigation system to preliminarily cancel the distortion from the distorted radio frequency pulse to produce a distortionless pulse at the correct pulse position as a distortion cancelled pulse. Otherwise, the distortion results produce various defects. By way of example, the distortion gives rise to errors in the measurements. For an aircraft approaching an airport, the error would result in grave consequences.
It is usual in a radio navigation system to use a sequence of pulse pairs or twin pulses as the modulating pulse. Each pulse pair consists of a first and a second modulating pulse in succession. The first and the second modulating pulses have the original pulse shape in common. A first and a second original radio frequency pulse are subjected to the multipath transmission. Such a radio frequency pulse pair is used in order to facilitate discrimination of the radio frequency signal used in the radio navigation system from other radio frequency signals or electromagnetic waves used in other systems. It is possible in connection with the radio frequency pulse pair to classify the undesired paths into near and distant paths which are near to the shortest path and distant therefrom, respectively. More particularly, the classification is based on the following facts. The first and the second original radio frequency pulses received through the shortest path, are distorted by the first and the second original radio frequency pulses, respectively, which are received through the near paths. The second original radio frequency pulse received through the shortest path, is distorted by the first original radio frequency pulse received through the distant paths. In this manner, the first and the second original radio frequency pulses are received as a first and a second distorted pulse of a first and a second distorted pulse shape, respectively. The first and the second distorted pulses have a first and a second distorted pulse position, respectively. In practice, a composite distortion results in the second distorted pulse as a result of reception of the radio frequency pulse pair through the shortest path, near paths, and distant paths.
The multipath transmission is similar in effect to echoes in a long-distance telephone network as, for example, in an international telephone network. An echo canceller is used in such a long-distance telephone network. The echo canceller is for use in cancelling the echoes by resorting to digital signal processing techniques in estimating the echoes. Although the situation is similar, it is impossible in practice to apply the principles of operation of the echo canceller to the multipath canceller. This is because a high frequency of the order of 1 GHz is used as the radio frequency in the radio navigation system. On resorting to the digital signal processing techniques, as high a sampling frequency as about 2 GHz or more must be used according to the Nyquist theorem. This high sampling frequency is impractical.